McCieLLanp— On Secondary Radiation. Nya 
the radium, and also to secondary radiation from the air traversed by the 
primary rays. 
Fifty milligrams of radium were used, enclosed in a vessel which stopped 
the a radiation. 
Liffect of the Position of the Plate. 
With the apparatus described, it is easy to detect a secondary radiation 
which travels in all directions from the part of the plate struck by the 
primary rays. The first point investigated was whether the amount of secondary 
radiation depended on the angle of incidence of the primary rays and on the angle 
that the testing-tube 7’ made with the plate ; whether, in fact, there was anything 
of the nature of ‘reflexion’ of the primary rays. This is a point which has been 
investigated by several observers using cathode rays as the primary rays; and 
while some observers have found a well-marked maximum of secondary radiation 
in the direction obeying the law of reflexion, others have not obtained such a 
result. 
The point was first tested by keeping the direction of the primary rays and 
that of the tube 7’ constant and at right angles, as in the figure, while the lead 
plate P was tilted; and this experiment showed a well-marked maximum when 6 
was 40°. 
The following observations were made, the secondary radiation being expressed 
in an arbitrary scale :— 
Secondary Radiation. 
9 = 224° we 
@ = 45°, eee 100 
EGON Team Serena Mustb a 
The observed numbers are reduced so as to express the maximum by 100. 
The plate P was then set so that ¢ was 45°, and this angle was kept constant, 
the direction of the primary rays only being changed. We then get as follows :— 
Secondary Radiation. 
€ 
9 = 28°, ; 
aM, 5 6 16% KO 
Qa@®, 5 6 « « 
@=8%, 2. «© 6 « 
In the third case the primary rays and the plate are both tilted, keeping the 
angle between them constant and equal to 45°, while the angle ¢ changes. We 
have then :— 
Secondary Radiation. 
ba, . o +» 1» & 
Pat, 5 2 6 5 iéo 
bai, «« o  s «6 » 8 
2G 2 
